AQA A-Level Chemistry AS Paper 1, June 2022: Question 22
1 mark · Easy difficulty · Multiple Choice
Identify the correct statement about the properties and reactions of magnesium and its compounds.
Practise this questionQuestion
Question text
22 Which statement is correct?
[1 mark]
Magnesium reacts with steam to give magnesium oxide as one of
A
the products.
B Magnesium acts as an oxidising agent in the extraction of titanium.
C Magnesium has a lower melting point than sodium.
D Magnesium hydroxide is very soluble in water.
Mark scheme
Show the mark scheme
22 A (AO1) 1 Magnesium oxide is a product when magnesium reacts with steam.
How to answer it
AQA A-Level Chemistry: Group 2 Trends & Reactions of Magnesium
This question assesses fundamental recall and conceptual understanding across AQA Specification Topic 3.2.2 (Group 2, the Alkaline Earth Metals) and Topic 3.1.3 (Bonding):
- The reaction of magnesium with steam vs cold water (products and conditions).
- The redox role of magnesium in the extraction of titanium from TiCl₄.
- Comparison of metallic bonding strengths across Period 3 elements.
- Solubility trends of Group 2 hydroxides in aqueous solution.
Multiple Choice Analysis
Identifying the single correct statement about magnesium
✅ Correct Answer: A
Magnesium reacts with steam to give magnesium oxide as one of the products.
The balanced equation is:
Mg(s) + H₂O(g) → MgO(s) + H₂(g)
Magnesium burns with a bright white flame in steam to produce solid white magnesium oxide (MgO) and hydrogen gas.
🧠 Exam Technique: Systematic Elimination
- Check Option A: Magnesium + steam gives MgO (with cold water it gives Mg(OH)₂). This is a well-known specification fact — immediately confirmed correct!
- Disprove Option B: In TiCl₄ + 2Mg → Ti + 2MgCl₂, Mg changes from oxidation state 0 to +2. Losing electrons means Mg is oxidised, making it a reducing agent, not an oxidising agent.
- Disprove Option C: Mg forms Mg²⁺ ions with 2 delocalised electrons per atom; Na forms Na⁺ with only 1 delocalised electron. Mg has a smaller ionic radius and higher charge density, leading to much stronger metallic bonding and a higher melting point (650 °C vs 98 °C).
- Disprove Option D: Hydroxide solubility increases down Group 2. Mg(OH)₂ is sparingly soluble (used as Milk of Magnesia/antacid precisely because its low solubility prevents it being caustic).
💡 Key Knowledge: Magnesium Reactions & Trends
- Mg with Cold Water (very slow):
Mg(s) + 2H₂O(l) → Mg(OH)₂(aq/s) + H₂(g) - Mg with Steam (rapid, vigorous):
Mg(s) + H₂O(g) → MgO(s) + H₂(g) - Titanium Extraction (Kroll process):
TiCl₄(l) + 2Mg(l) → Ti(s) + 2MgCl₂(s)
Mg acts as the reducing agent (Ti⁴⁺ + 4e⁻ → Ti). - Group 2 Hydroxide Solubility Trend:
Solubility increases down the group:
Mg(OH)₂ (sparingly soluble) → Ca(OH)₂ (slightly soluble) → Ba(OH)₂ (readily soluble).
❌ Common Errors & Pitfalls
- Confusing steam with cold water: Students often incorrectly state that steam produces Mg(OH)₂. Always remember that steam gives the oxide (MgO), while cold water gives the hydroxide (Mg(OH)₂).
- Agent inversion: Forgetting that a species that is oxidised acts as a reducing agent. Since Mg is oxidised from 0 to +2, it reduces Ti⁴⁺ to Ti.
- Sulfate vs Hydroxide solubility trends: Mixing up the two opposite trends:
• Group 2 Hydroxides: Solubility increases down the group.
• Group 2 Sulfates: Solubility decreases down the group (BaSO₄ is insoluble).
Topics
Inorganic Chemistry · 3.2.2 Group 2, The Alkaline Earth Metals · 3.2.1 Periodicity
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.